The virus in eastern China that since late February has killed 26 out of 128 confirmed cases has been officially named ‘avian influenza A (H7N9)’. Analysis of its genes shows a mixture derived from several bird flu viruses, and that the virus has been evolving for some time.

A century ago avian influenza viruses were called ‘fowl plague’, because they caused lethal infections in chickens. Their true nature was discovered in 1955 by Werner Schäfer, a German virologist and veterinarian who had spent the Second World War attending to the welfare of horses on the Eastern Front.

But H7N9 doesn’t cause fowl plague. If it infects chickens it does so without killing them or even making them sick.

How the victims became infected is still a mystery. The only substantial evidence linking them to chickens is that closing the Shanghai wet market (selling live birds) seemed to have a beneficial effect. Pigs don’t seem to carry the virus. But other animals might. A big hunt is on.

The failure of H5N1 to take off, or of swine flu to kill vast numbers in 2009-10, has caused pandemic fatigue. H7N9 is as nasty as H5N1 in that it is much more lethal for humans than ordinary flu. But it is much harder to monitor because it doesn’t cause disease in farm animals. Human cases in China have been diagnosed in eight provinces, Beijing and Shanghai, a vast area. They have popped up sporadically, and factors common to them have not been found. It is clear that the virus is very good at getting about quietly in its natural host, whichever species that is.

Pandemic potential? I am not fool enough to venture a quantitative prediction.